MRI method

ABSTRACT

A method of operating a magnetic resonance imaging scanner for imaging the heart of a patient comprising inducing apnoea in the patient; sensing an electrical heart waveform; in response thereto moving the chest wall of the patient to a desired location; and triggering the scanner to image.

The present invention relates to a method and apparatus for magneticresonance imaging and more particularly but not exclusively to amagnetic resonant imaging in coronary and vascular angiography.

Coronary artery disease is a major killer in Western Society. Evaluationof patients having suspected coronary artery disease is thereforedesirable. The conventional techniques include diagnosticcatheterization and X-ray-angiography. Both techniques have inherentrisk to the patient and have cost and time implications which arebecoming increasingly unacceptable.

It is desirable to use non-invasive techniques where possible. Apromising technique is magnetic resonance imaging. Such a techniquerequires successive data collection phases to generate successive sliceimages.

A major difficulty with magnetic resonance imaging of coronary arteriesis the need to perform a large number of imaging steps with a patientwho is likely to move, at least due to breathing.

It is normal to perform the imaging steps during breath holds of thepatient, but this does not solve the problem of variability as thepatient is likely to hold his breath at different chest positions witheach breath hold. To ameliorate the problems breath-coaching has beenperformed in an attempt to minimize variability of respiratory motion.Regardless of the success in terms of minimizing respiratory motion,such techniques are inconvenient to the patient and time-consuimg forboth the patient and medical staff.

Advances in software do allow for some movement between differentimaging steps but it is widely recognised that to achieve highresolution with minimal artefacts it would best to image with consistentchest position.

It would also be advantageous to perform the successive imaging stepswith the chest in its deflated state so as to minimize the separationbetween the chest and the heart.

According to a first aspect of the present invention there is provided amethod of operating a magnetic resonance imaging scanner for imaging aportion of the coronary arteries of a patient comprising:

-   -   inducing apnoea in the patient;    -   sensing an electrical heart waveform and,    -   in response thereto moving the chest wall of the patient to a        desired location and triggering the scanner to image.

Preferably the step of moving the chest wall of the patient to a desiredlocation comprises providing an enclosure around at least the chest ofthe patient and applying gas at a predetermined pressure to saidenclosure.

According to a second aspect of the present invention a method ofoperating a magnetic resonance imaging scanner for imaging a portion ofthe coronary arteries of a patient, comprising:

-   -   inducing apnoea in the patient;    -   sensing an electrical heart waveform;    -   in response thereto, applying pressure to the exterior of the        chest of the patient to produce expiration; and thereafter    -   triggering the scanner to image.

Preferably the method further comprises providing a structure forengaging at least the chest of the patient, said structure having asealing device for at least partially sealing to the chest, anddisposing said structure in engagement with said chest thereby definingwith said chest a pressure chamber, and wherein said step of inducingapnoea comprises applying cyclic pressure changes to said pressurechamber to cause hyperventilation of said patient.

Advantageously the step of applying cyclic pressure changes comprisesselecting a desired magnitude of pressure at a blower and successivelyreversing a connection of said blower to said chamber to therebysuccessively apply a positive pressure and a negative pressure.

Conveniently the method further comprises feeding back a pressure insaid chamber to control the output of said blower.

According to a third aspect of the present invention there is provided amagnetic resonance imaging device comprising a magnetic resonanceimaging scanner, a structure in use engaging at least the chest of apatient to define, with said chest, a pressure chamber, a respiratordevice for applying cyclically varying pressures to said chamber tocause cyclic forced respiration of said patient, said forced respirationcomprising successive inspiration and expiration periods, and pluralelectrodes connected to said respirator device and said magneticresonance imaging scanner for sensing an electrical waveform of theheart of said patient thereby triggering said magnetic resonance imagingscanner in accordance with-said waveform, and a discriminator device fortriggering said forced respiration in accordance with a predeterminedpoint of said waveform.

Preferably said discriminator device is operable to determine the timeperiod between successive maximum electrical amplitudes of saidwaveform, and further comprises variable timing circuitry for setting atriggering instant as a proportion of said time period.

Advantageously said discriminator device is operable to provide atrigger pulse in response to a predetermined characteristic of saidwaveform.

Conveniently said respirator device comprises a blower in use inducingair at an inlet and propelling said air from an outlet, a valve having avalve member and a body, said body having a first port connected to saidinlet, a second port connected to said outlet, a third exhaust portconnected to the ambient air and a fourth port connected to said chamberand a drive motor for moving said valve member with respect to said bodyfor selecting between a first state in which said first port isconnected to said fourth port and said second port is connected to saidthird exhaust port, and a second state in which said second port isconnected to said fourth port and said first port is connected to saidthird port.

Preferably said drive motor is further operable to move said valvemember to a third position is which said third port and said fourthports are connected together, and said first and second ports are bothclosed.

Conveniently said respirator device comprises control circuitry for saiddrive motor.

Advantageously said control circuitry comprises said discriminatordevice.

Preferably said respirator further comprises a blower drive motorconnected to said control circuitry whereby said control circuitry isoperable to set a desired output pressure from said blower.

Advantageously said respirator has a pressure feedback transducer havingan output-connected to said control circuitry for regulating thepressure output by said blower.

Preferably said control circuitry comprises a digital processor.

According to a fourth aspect of the present invention there is provideda ventilator for artificial respiration comprising a blower having anair inlet and an air outlet, a valve having a first port connected tosaid inlet, a second port connected to said outlet, a third portconnected as exhaust and a fourth port for connection to a structure inuse defining at least a part of a ventilator chamber for a chest of apatient, a first drive device for operating said blower and a seconddrive device for operating said valve and further comprising controlcircuitry for controlling said second drive device to provide cyclicconnections between said fourth port and said first and second ports,the ventilator yet further comprising a connection device for ECG leads,discrimination circuitry for detecting a selected electrical event atsaid connection device and in response thereto for supplying a controlsignal to said control circuitry for causing the operation of saidsecond drive device at a predetermined time relationship to saidelectrical event.

Preferably said discrimination circuitry is operable to determine thetime period between successive maximum electrical amplitudes of saidwaveform, and further comprises variable timing circuitry for setting atriggering instant as a proportion of said time period.

Advantageously said discrimination circuitry is operable to provide atrigger pulse in response to a predetermined characteristic of saidwaveform.

Preferably said respirator device comprises a blower in use inducing airat an inlet and-propelling said air from an outlet, a valve having avalve member and a body, said body having a first port connected to saidinlet, a second port connected to said outlet, a third exhaust portconnected to the ambient air and a fourth port connected to said chamberand a drive motor for moving said valve member with respect to said bodyfor selecting between a first state in which said first port isconnected to said fourth port and said second port is connected to saidthird exhaust port, and a second state in which said second port isconnected to said fourth port and said first port is connected to saidthird port.

Conveniently said drive motor is further operable to move said valvemember to a third position is which said third port and said fourthports are connected together, and said first and second ports are bothclosed.

Conveniently again said respirator device comprises control circuitryfor said drive motor.

Advantageously said control circuitry comprises said discriminatordevice.

Preferably the respirator further comprises a blower drive motorconnected to said control circuitry whereby said control circuitry isoperable to set a desired output pressure from said blower.

Preferably again said respirator has a pressure feedback input connectedto said control circuitry for regulating the pressure output by saidblower.

Advantageously said control circuitry comprises a digital processor.

An embodiment of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 shows a partial view of an MRI scanner with a patient connectedto an apparatus for artificial respiration in accordance with anembodiment of the invention;

FIG. 2 shows a first embodiment of a ventilator for producing artificialrespiration and controlling an MRI scanner; and

FIG. 3 shows a block schematic diagram of the ventilator of FIG. 2.

In the various figures, like reference numerals refer to like parts.

Referring first to FIG. 1 an MRI scanner 1 has a patient table 2supporting a patient 3 so as to move the patient between the coils 4 ofthe scanner. As is known to those skilled in the art, the coils 4typically comprise powerful magnets producing a B₀ field for the imagingprocedure, gradient coils for producing a gradient in the B₀ field inX,Y and Z directions and an RF coil for producing a B₁ field for spinrotation. Certain of the coils are toroidal so that the patient 3 issurrounded by the coils 4. The RF coil also detects the signal fromspins within the body.

A computer control unit 10 controls the position of the patient bymoving the table, controls the radio frequency supplied to the coilunits and pulses for the coil unit.

Secured to the patient's chest is a cuirass 20, which together withassociated sealing means forms a pressure chamber surrounding at leastthe chest of the patient. The cuirass is connected via large diametertubing 21 to a ventilator device 22 which is capable of providing pulsesof positive and negative pressure with respect to ambient under thecontrol of a control section 23. The ventilator device 22 has aconnection point 24 for ECG leads 25 which have electrodes 125 securedto the patient typically at 3 locations. Narrow diameter tubing 26 feedsback the pressure in the pressure chamber to the ventilator device 22,so that a desired pressure regime can be created, at a pressuretransducer input 26.

Referring now to FIG. 2, the ventilator in this embodiment consists of ablower 100 driven by an electric motor 101 and constantly moving airfrom an inlet port 102 to an outlet port 103. The inlet and outlet portsconnect to a valve 110 driven by an electric servo or stepper motor 111.The valve 110 has four ports, namely a positive pressure port 112, anegative pressure 113 an outlet port 114 and an exhaust port 115. Thepositive pressure port 112 is connected to the outlet port 103 of theblower 100. The negative pressure port 113 is connected to the inletport 102 of the blower 100. A valve member 120 has a first position inwhich it connects the low pressure port to the outlet port 114 and thehigh pressure port 112 to the exhaust 115, and a second position inwhich it connects the high pressure port 112 to the outlet port 114 andthe low pressure port 113 to the exhaust 115.

Preferably the valve includes further positions intermediate the firstand second positions and intermediate the second and first positions inwhich both ports 113 and 112 are closed and the outlet port 114 isconnected to the exhaust port 115.

The valve may be of the type described in our co-pending patentapplication No. PCT GB98/01317 or other valves may be used instead.

The outlet port 114 is connected to the cuirass 20 via the largediameter tubing 21.

The control section 23 receives power from a power lead 130 and has anoutput power lead 131 for driving the blower drive motor 101. In thepreferred embodiment this motor is speed-controlled by the controlsection 23 in accordance with pressure fed back to pressure transducerinput 26 and a pressure set-point.

The control section 23 has a second output lead 132 for controlling thevalve motor 111 preferably both in speed and position. The controlsection 23 contains processing circuitry so that the valve may operateto provide cyclically varying positive and negative pressure in thecuirass 20 (when sealingly engaged with the patient's chest), or may forexample maintain the valve in a single position for a substantial periodof time—for example—providing constant negative or constant positivepressure to the chest of a patient.

The control section, as previously discussed, is responsive to ECG leads25 connected to the control port 24 and further has an ECG triggeroutput line 135 for connection to an MRI scanner.

In an alternative embodiment, ECG leads and an ECG sensor in the MRIscanner are used to provide the necessary triggering.

Referring now to FIG. 3, a digital processor 300 is connected to aprogram memory 301 storing the application programs which are run on theprocessor. The processor is further connected to a display device 302which may be for example a liquid crystal display or a CRT. Controlinputs to the processor are from control keys 310 via a controlinterface 311 and an input line 312. The processor is connected to thecontrol port 24 for the ECG leads via an ECG interface 320 and an inputbus 322. The pressure transducer input 26 is connected to a pressuretransducer 330 which has an output connected to the processor via a bus332.

The processor 300 has three output buses, 340, 341, 342. The firstoutput bus 340 connects to a digital to analog converter 350 whichtypically comprises power switching circuitry and has an output formingthe output power lead 131. The second bus 341 feeds a stepper motorcontroller 351, connected to a motor driver 352 which has an output thatforms the second output lead 132. The third output bus 342 feeds a thirdinterface circuit 352 which has an output providing the ECG triggeroutput line 135. Alternatively the ECG trigger line may be coupleddirectly to the ECG interface 320.

Although the arrangement shown has separate buses 340, 341, 342 it willbe understood to those skilled in the art that a single bus could beprovided having multiplexed and interleaved functionality.

It is possible to provide multiple push-buttons, one for each forfunction, together with numeric keys. However, in the preferredembodiment a display screen is provided with pressure lamp andselectors.

In a first mode of operation the respirator shown in FIG. 2 may provideforced respiration at a rate related to the heart rate of the patient.Thus, the chest may be arranged to be compressed at the time the heartbeats so as to provide assistance to the heart.

This is achieved by the processing circuitry which operates to averagethe time between R wave peaks and a keypad 136 which allows theselection of a desired percentage of that time, whereupon the processingcircuitry causes the valve 110 to output a positive pressure at theselected instant.

The control section 23 is also operable to set the valve to providelarge amplitude pressure changes at a rate which causes the patient tobecome hyperventilated.

The technique for using the device is as follows:—

Firstly the respirator 22 is operated cyclically to move the chest withsufficient amplitude, and sufficiently often to cause hyperventilation.This induces apnoea—ie. it removes the body's trigger to breathe. Thisamplitude and frequency is achieved by sensing the peak value ofpressure in the pressure line 426 and using the processor to vary theblower speed to provide the desired pressure.

Once apnoea is achieved, the respirator 22 is switched to aheart-triggered mode in which the movement of the chest is triggered byan event on the ECG waveform. In one technique, the discriminatorcircuitry in the respirator monitors the electrical activity of theheart and determines the timing between the peaks of the R waves of theECG and a control is then operated to set a trigger instant forbreathing at a desired percentage of the time period between successiveR wave peaks.

It will be understood that breathing may be triggered with eachheartbeat or alternatively a breathing rate corresponding to every two,three or four or more heartbeats may be selected as desired.

Given the prior hyperventilation, there is no longer a desire to breathand it is possible to apply external pressure to the chest to produce an“expiration hold” at a time when it is required to trigger the MRIscanner. This results in the heart being near to the chest wall whichprovides good results due to minimizing the distance within the body forimaging to occur. It is also possible to hold the chest at any desiredposition, including maximum inhalation, neutral or any intermediateposition.

Due to the imposed external pressure and the previous hyperventilation,it is possible to hold the chest in the expiration position for, forexample, 30 heartbeats, followed by a breathing cycle and then return tothe exact location of the previous hold.

There are a number of advantages to the technique of the invention.Typically, using techniques in which the patient needs to co-operate, itmay take more than 60 minutes to image the coronary arteries of thepatient. Given the high cost of MRI scanners and of the staff requiredto operate them, this is clearly unacceptable. It is also undesirable tothe patient and in any event the resolution of the scan may beinsufficient to allow doctors to form an accurate diagnosis. Bycomparison it has been found possible to obtain high resolution imagessufficient for good diagnostic quality using the present invention inless than 10 minutes. This has clear advantages to the comfort of thepatient who in any event does not experience discomfort as would becaused by the need to hold breath. From the viewpoint of the hospitalthe speed of the technique provides a highly cost-effective procedureand the quality of the images is advantageous to the physicians who areforming a diagnosis.

In a second embodiment, the discriminator may respond to a particularwaveform shape in the electrical heart waveform allowing directtriggering in response to the event which gave rise to that shape.

It will therefore be seen that the present invention by avoiding the twomajor disadvantages of the patient-cooperation techniques (patientbreathing out at an inopportune time and a patient breathinginconsistently from breath to breath) the present invention allows forrapid high resolution imaging of coronary arteries.

As discussed above, the respirator used in the present invention mayemploy a cuirass sealingly engaging the chest of a patient, with airconnections to a valve-blower arrangement. It will be known to thoseskilled in the art that conventional MRI techniques involve placing of amagnetic coil (for example having a figure-8 shape) on the chest of thepatient and then introduction of the patient into a set of imagingcoils. The conventional MRI scanners have a small clearance around thepatient to provide the best, field distribution. The use of a knowncuirass may provide difficulties due to insufficient clearance andinability to adequately house the chest coil. Thus, in accordance with afurther aspect of the present invention a cuirass is provided having airinlet ports to the side of the chest and having a generally raisedportion over the upper part of the chest to house the coil. The sealwhich is preferably a seal as defined in our co-pending patentapplication may have a cut-out portion around its edge to allow forpassage of the cabling to the coil, the dimension of the cut-out beingadapted to that of the cabling so as to ensure an adequate seal.

1. A method of operating a magnetic resonance imaging scanner forimaging the heart of a patient comprising: providing a structure forengaging at least the chest of the patient, said structure having asealing device for at least partially sealing to said chest, anddisposing said structure in engagement with said chest thereby definingwith said chest a pressure chamber; inducing apnoea in the patient byapplying cyclic pressure changes to said pressure chamber to causehyperventilation of said patient; sensing an electrical heart waveform;moving the chest wall of the patient to a desired location in responseto said electrical heart waveform; and triggering the scanner to image.2. The method of claim 1 wherein said step of applying cyclic pressurechanges comprises selecting a desired magnitude of pressure at a blowerand successively reversing a connection of said blower to said chamberto thereby successively apply a positive pressure and a negativepressure.
 3. The method of claim 2 further comprising feeding back apressure in said chamber to control the output of said blower.
 4. Amagnetic resonance imaging device comprising: a magnetic resonanceimaging scanner; a structure adapted to engage at least the chest of apatient to define, with said chest, a pressure chamber; a respiratordevice for applying cyclically varying pressures to said chamber tocause cyclic forced respiration of said patient, said forced respirationcomprising successive inspiration and expiration periods; pluralelectrodes connected to said respirator device and said magneticresonance imaging scanner for sensing an electrical waveform of theheart of said patient, and triggering said magnetic resonance imagingscanner in accordance with said waveform; and a discriminator device fortriggering said forced respiration in synchronism with a predeterminedpoint of said waveform wherein said respirator device comprises a blowerin use inducing air at an inlet and propelling said air from an outlet,a valve having a valve member and a body, said body having a first portconnected to said inlet, a second port connected to said outlet, a thirdexhaust port connected to the ambient air and a fourth port connected tosaid chamber and a drive motor for moving said valve member with respectto said body for selecting between a first state in which said firstport is connected to said fourth port and said second port is connectedto said third exhaust port, and a second state in which said second portis connected to said fourth port and said first port is connected tosaid third port.
 5. The magnetic resonance imaging device of claim 4wherein said drive motor is further operable to move said valve memberto a third position is which said third port and said fourth ports areconnected together, and said first and second ports are both closed. 6.The magnetic resonance imaging device of claim 5 wherein said respiratordevice comprises control circuitry for said drive motor.
 7. The magneticresonance imaging device of claim 6 wherein said control circuitrycomprises said discriminator device.
 8. The magnetic resonance imagingdevice of claim 7 wherein said respirator further comprises a blowerdrive motor connected to said control circuitry whereby said controlcircuitry is operable to set a desired output pressure from said blower.9. The magnetic resonance imaging device of claim 8 wherein saidrespirator has a pressure feedback transducer having an output connectedto said control circuitry for regulating the pressure output by saidblower.
 10. The magnetic resonance imaging device of claim 6 whereinsaid control circuitry comprises a digital processor.
 11. A ventilatorfor artificial respiration comprising a blower having an air inlet andan air outlet, a valve having a valve body and a valve member, the valvebody having a first port connected to said inlet, a second portconnected to said outlet, a third port connected to the ambient air anda fourth port for connection to a structure in use defining at least inpart a ventilator chamber, the valve member having a first position inwhich said first and third ports are connected together and said secondand fourth ports are connected together, and a second position in whichsaid second and third ports are connected together and said first andfourth ports are connected together, the ventilator further comprising afirst drive motor for operating said blower, a second drive motor foroperating said valve and control circuitry for controlling said seconddrive motor to move said valve member between said first and secondpositions, the ventilator yet further comprising a connection device forelectro-cardiograph leads and discrimination circuitry for determining aselected electrical event at said connection device and in responsethereto for supplying a control signal to said control circuitry formoving said valve member to said first position at a predetermined timerelationship to said electrical event, said discrimination circuitryhaving an output for connection to a magnetic resonance imaging devicefor triggering said device in accordance with a desired electricalcondition at said connection device.
 12. The ventilator of claim 11wherein said discrimination circuitry is operable to determine the timeperiod between successive maximum electrical amplitudes of saidwaveform, and further comprises variable timing circuitry for setting atrigger instant as a proportion of said time period.
 13. The ventilatorof claim 11 wherein said discrimination circuitry is operable to providea trigger pulse in response to a predetermined characteristic of saidwaveform.
 14. The ventilator of claim 11 wherein said respirator devicecomprises a blower in use inducing air at an inlet and propelling saidair from an outlet, a valve having a valve member and a body, said bodyhaving a first port connected to said inlet, a second port connected tosaid outlet, a third exhaust port connected to the ambient air and afourth port connected to said chamber and a drive motor for moving saidvalve member with respect to said body for selecting between a firststate in which said first port is connected to said fourth port and saidsecond port is connected to said third exhaust port, and a second statein which said second port is connected to said fourth port and saidfirst port is connected to said third port.
 15. The ventilator of claim14 wherein said drive motor is further operable to move said valvemember to a third position is which said third port and said fourthports are connected together, and said first and second ports are bothclosed.
 16. The ventilator of claim 15 wherein said respirator devicecomprises control circuitry for said drive motor.
 17. The ventilator ofclaim 16 wherein said control circuitry comprises said discriminatordevice.
 18. The ventilator of claim 17 wherein said respirator furthercomprises a blower drive motor connected to said control circuitrywhereby said control circuitry is operable to set a desired outputpressure from said blower.
 19. The ventilator of claim 18 wherein saidrespirator has a pressure feedback input connected to said controlcircuitry for regulating the pressure output by said blower.
 20. Theventilator of claim 16 wherein said control circuitry comprises adigital processor.